The invention concerns a magnet coil system that is at least partially superconducting at a cryogenic temperature, comprising at least two serially connected partial coils which are each bridged by a superconducting switch, such that the partial coils form independent current loops when the superconducting switches are closed.
A magnet coil system of this type is disclosed in US 2006/0066429 A1.
By connecting several superconducting coils in series, wherein each coil is bridged by a superconducting switch, the coils can be sequentially charged with individual currents via one single current source and one single pair of current supply lines. When the coils are magnetically decoupled from each other, each coil can be directly charged with its current. This principle is normally used for charging shim coils which are designed such that they do not couple with each other. If this is not the case, the couplings must be taken into consideration, and a charging sequence must be observed.
The same principle is also used for charging partial coils of a magnet system with different currents. This is of particular interest when a magnet system comprises partial coils of conventional superconductors (LTS) and of high-temperature superconductors (HTS). Use of different currents in the various partial coils yields additional degrees of freedom for the system design. Since the available dimensions for HTS are limited, one is dependent to these degrees of freedom for designing an optimum system.
US2006/066429A1 utilizes two electric loops in order to obtain one pure HTS and one pure LTS electric loop. This avoids problematic superconducting connections between HTS and LTS, and a system with little drift can be realized. The technology for superconducting connections between similar conductors exists. Moreover, the above-mentioned additional degrees of freedom for the design can also be realized by using two electric loops.
In particular, when an HTS is used for superconducting magnet systems that are used e.g. for high-resolution nuclear magnetic resonance experiments, compensation of a slow current loss (field drift) is required. When an HTS coil is connected in series with an LTS coil, the connections between HTS and LTS (HTS-LTS joints) are often problematic and produce a field drift. If the HTS coil has its own current path, the inductance is typically very small. Even minor problems with wires or superconducting connections produce a large field drift, in particular, since the HTS coil substantially contributes to the field. An HTS often has a smaller so-called n-value compared to an LTS. The n-value describes the behavior of the resistance R of a superconductor in dependence on the current I that flows through the superconductor, which will be explained below. The smaller n-value of HTS means that a load will result in a higher field drift that must be compensated for.
Drift compensation with LTS coils may also be desirable. The current load on the wires can be increased, wherein, in turn, more compact and less expensive systems can be realized. Moreover, expensive repair work can be avoided in case of imperfect superconducting connections or damaged wires.
The suitable means for such drift compensation is a flux pump. Drift compensation using flux pumps is currently the only possibility of compensating the drift in the “persistent mode” on a long-term and continuous basis, i.e. without having to permanently guide the magnetic current into the cryostat, which would be the case for operating a magnet coil system in the “driven mode”.
It is also desirable to use two independent electric loops in order to obtain additional degrees of freedom for the design. Utilization of the superconductors is thereby improved and consequently, more compact and less expensive systems can finally be realized. In particular, HTS are only available in a few dimensions, and the degrees of freedom of different currents and different coil sections are required to obtain an effective design.
If both two independent electric loops as well as drift compensation are required, each electric circuit could be provided with one flux pump. This involves great expense, requires a large amount of space in the cryostat and is also susceptible to failure due to the increased complexity. Both flux pumps would then have to be controlled and field control would be complex. In particular, it would be difficult to determine which partial coil produces which part of the field loss. This determination is, however, necessary in order to correctly control the performance of both flux pumps and keep the individual currents constant. The use of two flux pumps would, however, most likely result in uncontrolled current shifts from one electric circuit to the other. In order to prevent this, the currents of the partial coils would have to be measured, which is relatively demanding, e.g. using Hall probes inside a measuring winding slightly outside of the magnet. It is, however, doubtful whether this measurement is sufficiently precise. In total, this possibility of drift compensation does not seem to be practicable.
It is therefore the underlying purpose of the invention to propose a magnet coil system comprising two independent electric circuits, wherein effective drift compensation can be performed in a simple manner.